Web Security C. Evans
Internet-Draft C. Palmer
Intended status: Standards Track R. Sleevi
Expires: January 13, 2014 Google, Inc.
July 12, 2013
Public Key Pinning Extension for HTTPdraft-ietf-websec-key-pinning-08
Abstract
This memo describes an extension to the HTTP protocol allowing web
host operators to instruct user agents (UAs) to remember ("pin") the
hosts' cryptographic identities for a given period of time. During
that time, UAs will require that the host present a certificate chain
including at least one Subject Public Key Info structure whose
fingerprint matches one of the pinned fingerprints for that host. By
effectively reducing the number of authorities who can authenticate
the domain during the lifetime of the pin, pinning may reduce the
incidence of man-in-the-middle attacks due to compromised
Certification Authorities.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 13, 2014.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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Internet-Draft Public Key Pinning Extension for HTTP July 20131. Introduction
We propose a new HTTP header to enable a web host to express to user
agents (UAs) which Subject Public Key Info (SPKI) structure(s) UAs
SHOULD expect to be present in the host's certificate chain in future
connections using TLS (see [RFC5246]). We call this "public key
pinning". At least one UA (Google Chrome) has experimented with the
idea by shipping with a user-extensible embedded set of pins.
Although effective, this does not scale. This proposal addresses the
scale problem.
Deploying public key pinning safely will require operational and
organizational maturity due to the risk that hosts may make
themselves unavailable by pinning to a SPKI that becomes invalid.
(See Section 4.) We believe that, with care, host operators can
greatly reduce the risk of MITM attacks and other false-
authentication problems for their users without incurring undue risk.
We intend for hosts to use public key pinning together with HSTS
([RFC6797]), but is possible to pin keys without requiring HSTS.
This draft is being discussed on the WebSec Working Group mailing
list, websec@ietf.org.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Server and Client Behavior2.1. Response Header Field Syntax
The Public-Key-Pins HTTP response header field (PKP header field)
indicates to a UA that it should perform Pin Validation (Section 2.6)
in regards to the host emitting the response message containing this
header field, and provides the necessary information for the UA to do
so.
Figure 1 describes the ABNF (Augmented Backus-Naur Form) syntax of
the header field. It is based on the Generic Grammar defined in
Section 2 of [RFC2616] (which includes a notion of "implied linear
whitespace", also known as "implied *LWS").
Public-Key-Pins =
"Public-Key-Pins" ":" [ directive ] *( ";" [ directive ] )
Public-Key-Pins-Report-Only =
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"Public-Key-Pins-Report-Only" ":" [ directive ] *( ";" [ directive ] )
directive = simple-directive
/ pin-directive
simple-directive = directive-name [ "=" directive-value ]
directive-name = token
directive-value = token
/ quoted-string
pin-directive = "pin-" token "=" quoted-string
Figure 1: HPKP Header Syntax
token and quoted-string are used as defined in [RFC2616],
Section 2.2.
The directives defined in this specification are described below.
The overall requirements for directives are:
1. The order of appearance of directives is not significant.
2. All simple-directives MUST appear only once in a PKP header
field. Directives are either optional or required, as stipulated
in their definitions.
3. Directive names are case-insensitive.
4. UAs MUST ignore any PKP header fields containing directives, or
other header field value data, that do not conform to the syntax
defined in this specification.
5. If a PKP header field contains any directive(s) the UA does not
recognize, the UA MUST ignore the those directives.
6. If the PKP header field otherwise satisfies the above
requirements (1 through 5), the UA MUST process the directives it
recognizes.
Additional directives extending the semantic functionality of the PKP
header field can be defined in other specifications, with a registry
(having an IANA policy definition of IETF Review [RFC2616]) defined
for them at such time. Such future directives will be ignored by UAs
implementing only this specification, as well as by generally non-
conforming UAs.
In the pin-directive, the token is the name of a cryptographic hash
algorithm, and MUST be either "sha1" or "sha256". The quoted-string
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is a sequence of base 64 digits: the base 64-encoded SPKI Fingerprint
([RFC4648]). See Section 2.4.
2.1.1. The max-age Directive
The REQUIRED "max-age" directive specifies the number of seconds,
after the reception of the PKP header field, during which the UA
SHOULD regard the host (from whom the message was received) as a
Known Pinned Host. The delta-seconds production is specified in
[RFC2616].
The syntax of the max-age directive's REQUIRED value (after quoted-
string unescaping, if necessary) is defined as:
max-age-value = delta-seconds
delta-seconds = 1*DIGIT
Figure 2: max-age Value Syntax
delta-seconds is used as defined in [RFC2616], Section 3.3.2.
NOTE: A max-age value of zero (i.e., "max-age=0") signals the UA to
cease regarding the host as a Known Pinned Host, including the
includeSubDomains directive (if asserted for that Known Pinned Host).
See Section 2.3.1.
2.1.2. The includeSubDomains Directive
The OPTIONAL "includeSubDomains" directive is a valueless directive
which, if present (i.e., it is "asserted"), signals to the UA that
the Pinning Policy applies to this Pinned Host as well as any
subdomains of the host's domain name.
2.1.3. The report-uri Directive
The OPTIONAL "report-uri" directive indicates the URI to which the UA
SHOULD report Pin Validation failures (Section 2.6). The UA POSTs
the reports to the given URI as described in Section 3.
When used in the Public-Key-Pins-Report-Only header, the UA SHOULD
POST reports for Pin Validation failures to the indicated report-uri,
although the UA MUST NOT enforce Pin Validation. That is, in the
event of Pin Validation failure when the host has set the Public-Key-
Pins-Report-Only header, the UA performs Pin Validation only to check
whether or not it should POST a report, but not for causing
connection failure.
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If a Host sets both the Public-Key-Pins header and the Public-Key-
Pins-Report-Only header, the UA MUST NOT enforce Pin Validation, and
MUST note only the pins and directives given in the Public-Key-Pins-
Report-Only header.
When used in the Public-Key-Pins header, the presence of a report-uri
directive indicates to the UA that the UA MUST enforce Pin
Validation, and the UA SHOULD also, in the event of Pin Validation
failure, POST a report to the report-uri.
Note that the report-uri need not necessarily be in the same Internet
domain or web origin as the Known Pinned Host.
Hosts may set report-uris that use HTTP, HTTPS, or other schemes. If
the scheme in the report-uri is HTTPS, UAs MUST perform Pinning
Validation when the host in the report-uri is a Known Pinned Host;
similarly, UAs MUST apply HSTS if the host in the report-uri is a
Known HSTS Host.
UAs SHOULD make their best effort to report Pin Validation failures
to the report-uri, but may fail to report in exceptional conditions.
For example, if connecting the report-uri itself incurs a Pinning
Validation failure or other certificate validation failure, the UA
MUST cancel the connection (and MAY attempt to re-send the report
later). Similarly, if Known Pinned Host A sets a report-uri
referring to Known Pinned Host B, and if B sets a report-uri
referring to A, and if both hosts fail Pin Validation, the UA SHOULD
detect and break the loop by failing to send reports to and about
those hosts.
UAs SHOULD limit the rate at which they send reports. For example,
it is unnecessary to send the same report to the same report-uri more
than once.
2.1.4. The strict Directive
The OPTIONAL "strict" directive is a valueless directive which, if
present (i.e., it is "asserted"), signals to the UA that it should
apply to the Pinned Host the Pinning Policy expressed in the PKP
header exactly as specified, ignoring local client policy.
2.1.5. Examples
Figure 3 shows some example response header fields using the pins
extension (folded for clarity).
Public-Key-Pins: max-age=3000;
pin-sha1="4n972HfV354KP560yw4uqe/baXc=";
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pin-sha1="IvGeLsbqzPxdI0b0wuj2xVTdXgc="
Public-Key-Pins: max-age=2592000;
pin-sha1="4n972HfV354KP560yw4uqe/baXc=";
pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ="
Public-Key-Pins: max-age=2592000;
pin-sha1="4n972HfV354KP560yw4uqe/baXc=";
pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=";
report-uri="http://example.com/pkp-report"
Public-Key-Pins-Report-Only: max-age=2592000;
pin-sha1="4n972HfV354KP560yw4uqe/baXc=";
pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=";
report-uri="http://example.com/pkp-report"
Public-Key-Pins: pin-sha1="4n972HfV354KP560yw4uqe/baXc=";
pin-sha1="qvTGHdzF6KLavt4PO0gs2a6pQ00=";
pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=";
max-age=259200
Public-Key-Pins: pin-sha1="4n972HfV354KP560yw4uqe/baXc=";
pin-sha1="qvTGHdzF6KLavt4PO0gs2a6pQ00=";
pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=";
max-age=10000; includeSubDomains
Figure 3: HPKP Header Examples
2.2. Server Processing Model
This section describes the processing model that Pinned Hosts
implement. The model comprises two facets: the processing rules for
HTTP request messages received over a secure transport (e.g. TLS
[RFC5246]); and the processing rules for HTTP request messages
received over non-secure transports, such as TCP.
2.2.1. HTTP-over-Secure-Transport Request Type
When replying to an HTTP request that was conveyed over a secure
transport, a Pinned Host SHOULD include in its response exactly one
PKP header field that MUST satisfy the grammar specified above in
Section 2.1. If the Pinned Host does not include the PKP header
field, and if the connection passed Pin Validation, UAs MUST treat
the host as if it had set its max-age to 0 (see Section 2.3.1).
Establishing a given host as a Known Pinned Host, in the context of a
given UA, MAY be accomplished over the HTTP protocol, which is in
turn running over secure transport, by correctly returning (per this
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specification) at least one valid PKP header field to the UA. Other
mechanisms, such as a client-side pre-loaded Known Pinned Host list
MAY also be used.
2.2.2. HTTP Request Type
Pinned Hosts SHOULD NOT include the PKP header field in HTTP
responses conveyed over non-secure transport. UAs MUST ignore any
PKP header received in an HTTP response conveyed over non-secure
transport.
2.3. User Agent Processing Model
This section describes the HTTP Public Key Pinning processing model
for UAs.
The UA processing model relies on parsing domain names. Note that
internationalized domain names SHALL be canonicalized according to
the scheme in Section 10 of [RFC6797].
2.3.1. Public-Key-Pins Response Header Field Processing
If the UA receives, over a secure transport, an HTTP response that
includes a PKP header field conforming to the grammar specified in
Section 2.1, and there are no underlying secure transport errors or
warnings (see Section 2.5), the UA MUST either:
o Note the host as a Known HSTS Host if it is not already so noted
(see Section 2.3.2),
or,
o Update the UA's cached information for the Known Pinned Host if
any of of the max-age, includeSubDomains, strict, or report-uri
header field value directives convey information different than
that already maintained by the UA.
o The max-age value is essentially a "time to live" value relative
to the time of the most recent observation of the PKP header
field.
o If the max-age header field value token has a value of 0, the UA
MUST remove its cached Pinning Policy information (including the
includeSubDomains and strict directives, if asserted) if the
Pinned Host is Known, or, MUST NOT note this Pinned Host if it is
not yet Known.
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o If a UA receives more than one PKP header field in an HTTP
response message over secure transport, then the UA MUST process
only the first such header field.
Otherwise:
o If the UA receives the HTTP response over insecure transport, or
if the PKP header is not a Valid Pinning Header (see Section 2.5),
the UA MUST ignore any present PKP header field(s).
o The UA MUST ignore any PKP header fields not conforming to the
grammar specified in Section 2.1.
2.3.2. Noting a Pinned Host - Storage Model
If the substring matching the host production from the Request-URI
(of the message to which the host responded) syntactically matches
the IP-literal or IPv4address productions from Section 3.2.2 of
[RFC3986], then the UA MUST NOT note this host as a Known Pinned
Host.
Otherwise, if the substring does not congruently match a Known Pinned
Host's domain name, per the matching procedure specified in
Section 8.2 of [RFC6797], then the UA MUST note this host as a Known
Pinned Host, caching the Pinned Host's domain name and noting along
with it the time of the observation (also known as the Effective Pin
Date), the value of the max-age directive, whether or not the
includeSubDomains or strict directives are asserted, the value of the
report-uri directive (if present), and any other metadata from
optional or future PKP header directives.
UAs MAY set an upper limit on the value of max-age, so that UAs that
have noted erroneous pins (whether by accident or due to attack) have
some chance of recovering over time. If the server sets a max-age
greater than the UA's upper limit, the UA MAY behave as if the server
set the max-age to the UA's upper limit. For example, if the UA caps
max-age at 5184000 seconds (60 days), and a Pinned Host sets a max-
age directive of 90 days in its Valid Pinning Header, the UA MAY
behave as if the max-age were effectively 60 days. (One way to
achieve this behavior is for the UA to simply store a value of 60
days instead of the 90 day value provided by the Pinned Host.) For
UA implementation guidance on how to select a maximum max-age, see
Section 4.1.
The UA MUST NOT modify any pinning metadata of any superdomain
matched Known Pinned Host.
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A Known Pinned Host is "expired" if the Effective Pin Date plus the
max-age refers to a date in the past. The UA MUST ignore all expired
Known Pinned Hosts from its cache if, at any time, an expired Known
Pinned Host exists in the cache.
2.3.3. HTTP-Equiv <Meta> Element Attribute
UAs MUST NOT heed http-equiv="Public-Key-Pins" attribute settings on
<meta> elements [W3C.REC-html401-19991224] in received content.
2.4. Semantics of Pins
An SPKI Fingerprint is defined as the output of a known cryptographic
hash algorithm whose input is the DER-encoded ASN.1 representation of
the SubjectPublicKeyInfo (SPKI) field of an X.509 certificate. A Pin
is defined as the combination of the known algorithm identifier and
the SPKI Fingerprint computed using that algorithm.
The SPKI Fingerprint is encoded in base 64 for use in an HTTP header.
(See [RFC4648].)
In this version of the specification, the known cryptographic hash
algorithms are SHA-1, identified as "sha1", and SHA-256, identified
as "sha256" ([RFC4634]). (Future versions of this specification may
add new algorithms and deprecate old ones.) UAs MUST ignore Pins for
which they do not recognize the algorithm identifier. UAs MUST
continue to process the rest of a PKP response header field and note
Pins for algorithms they do recognize; UAs MUST recognize "sha1" and
"sha256".
Figure 4 reproduces the definition of the SubjectPublicKeyInfo
structure in [RFC5280].
SubjectPublicKeyInfo ::= SEQUENCE {
algorithm AlgorithmIdentifier,
subjectPublicKey BIT STRING }
AlgorithmIdentifier ::= SEQUENCE {
algorithm OBJECT IDENTIFIER,
parameters ANY DEFINED BY algorithm OPTIONAL }
Figure 4: SPKI Definition
If the SubjectPublicKeyInfo of a certificate is incomplete when taken
in isolation, such as when holding a DSA key without domain
parameters, a public key pin cannot be formed.
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We pin public keys, rather than entire certificates, to enable
operators to generate new certificates containing old public keys
(see [why-pin-key]).
See Appendix A for an example non-normative program that generates
SPKI Fingerprints from SubjectPublicKeyInfo fields in certificates.
2.5. Noting Pins
Upon receipt of the Public-Key-Pins response header field, the UA
notes the host as a Pinned Host, storing the Pins and their
associated directives in non-volatile storage (for example, along
with the HSTS metadata). The Pins and their associated directives
are collectively known as Pinning Metadata.
The UA MUST observe these conditions when noting a Host:
o The UA MUST note the Pins if and only if it received the Public-
Key-Pins response header field over an error-free TLS connection.
If the host is a Pinned Host, this includes the validation added
in Section 2.6.
o The UA MUST note the Pins if and only if the TLS connection was
authenticated with a certificate chain containing at least one of
the SPKI structures indicated by at least one of the given SPKI
Fingerprints. (See Section 2.6.)
o The UA MUST note the Pins if and only if the given set of Pins
contains at least one Pin that does NOT refer to an SPKI in the
certificate chain. (That is, the host must set a Backup Pin; see
Section 4.3.)
If the Public-Key-Pins response header field does not meet all three
of these criteria, the UA MUST NOT note the host as a Pinned Host. A
Public-Key-Pins response header field that meets all these critera is
known as a Valid Pinning Header.
Whenever a UA receives a Valid Pinning Header, it MUST set its
Pinning Metadata to the exact Pins, max-age, and (if any) report-uri
and strict mode given in the most recently received Valid Pinning
Header.
For forward compatibility, the UA MUST ignore any unrecognized
Public-Key-Pins header directives, while still processing those
directives it does recognize. Section 2.1 specifies the directives
max-age, pins, includeSubDomains, report-uri, and strict, but future
specifications and implementations might use additional directives.
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Internet-Draft Public Key Pinning Extension for HTTP July 20132.6. Validating Pinned Connections
When a UA connects to a Pinned Host, if the TLS connection has
errors, the UA MUST terminate the connection without allowing the
user to proceed anyway. (This behavior is the same as that required
by [RFC6797].)
If the connection has no errors, then the UA will determine whether
to apply a new, additional correctness check: Pin Validation. A UA
SHOULD perform Pin Validation whenever connecting to a Known Pinned
Host, but MAY allow Pin Validation to be disabled for Hosts according
to local policy. For example, a UA may disable Pin Validation for
Pinned Hosts whose validated certificate chain terminates at a user-
defined trust anchor, rather than a trust anchor built-in to the UA.
However, if the Pinned Host Metadata indicates that the Pinned Host
is operating in "strict mode" (see Section 2.1.4), then the UA MUST
perform Pin Validation.
To perform Pin Validation, the UA will compute the SPKI Fingerprints
for each certificate in the Pinned Host's validated certificate
chain, using each supported hash algorithm for each certificate.
(For the purposes of Pin Validation, the UA MUST ignore certificates
whose SPKI cannot be taken in isolation, and MUST ignore superfluous
certificates in the chain that do not form part of the validating
chain.) The UA will then check that the set of these SPKI
Fingerprints intersects the set of SPKI Fingerprints in that Pinned
Host's Pinning Metadata. If there is set intersection, the UA
continues with the connection as normal. Otherwise, the UA MUST
treat this Pin Failure as a non-recoverable error. Any procedure
that matches the results of this Pin Validation procedure is
considered equivalent.
Note that, although the UA has previously received Pins at the HTTP
layer, it can and MUST perform Pin Validation at the TLS layer,
before beginning an HTTP conversation over the TLS channel. The TLS
layer thus evaluates TLS connections with pinning information the UA
received previously, regardless of mechanism: statically preloaded,
via HTTP header, or some other means (possibly in the TLS layer
itself).
2.7. Interactions With Preloaded Pin Lists
UAs MAY choose to implement additional sources of pinning
information, such as through built-in lists of pinning information.
Such UAs SHOULD allow users to override such additional sources,
including disabling them from consideration.
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UAs that support additional sources of pinning information MUST use
the most recently observed pinning information when performing Pin
Validation for a host. The most recently observed pinning
information is determined based upon the most recent Effective Pin
Date, as described in Section 2.3.2.
If the result of noting a Valid Pinning Header is to disable pinning
for the host, such as through supplying a max-age directive with a
value of 0, UAs MUST allow this new information to override any other
pinning data. That is, a host must be able to un-pin itself, even in
the presence of built-in pins.
Example: A UA may ship with a pre-configured list of pins that are
collected from past observations of Valid Pinning Headers supplied by
hosts. In such a solution, the pre-configured list should track when
the Valid Pinning Header was last observed, in order to permit site
operators to later update the value by supplying a new Valid Pinning
Header. Updates to such a pre-configured list should not update the
Effective Pin Dates for each host unless the list vendor has actually
observed a more recent header. This is to prevent situations where
updating the Effective Pin Date on a pre-configured list of pins may
effectively extend the max-age beyond the site operator's stated
policy.
Example: A UA may ship with a pre-configured list of pins that are
collected through out-of-band means, such as direct contact with the
site operator. In such a solution, the site operator accepts
responsibility for keeping the configured Valid Pinning Header in
sync with the vendor's list, allowing the UA vendor to have each
update to the list be treated as as an update of the Effective Pin
Date.
2.8. Pinning Self-Signed End Entities
If UAs accept hosts that authenticate themselves with self-signed end
entity certificates, they MAY also allow hosts to pin the public keys
in such certificates. The usability and security implications of
this practice are outside the scope of this specification.
3. Reporting Pin Validation Failure
When a Known Pinned Host has set the report-uri directive, the UA
SHOULD report Pin Validation failures to the indicated URI. The UA
does this by POSTing a JSON ([RFC4627]) message to the URI; the JSON
message takes this form:
{
"date-time": date-time,
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"hostname": hostname,
"port": port,
"certificate-chain": [
pem1, ... pemN
],
"known-pins": [
known-pin1, ... known-pinN
]
}
Figure 5: JSON Report Format
Whitespace outside of quoted strings is not significant. The key/
value pairs may appear in any order, but each SHOULD appear only
once.
The date-time indicates the time the UA observed the Pin Validation
failure. It is provided as a string formatted according to
Section 5.6, "Internet Date/Time Format", of [RFC3339].
The hostname is the hostname to which the UA made the original
request that failed Pin Validation. It is provided as a string.
The port is the port to which the UA made the original request that
failed Pin Validation. It is provided either as a string or as an
integer.
The certificate-chain is the certificate chain, as constructed by the
UA during certificate chain verification. (This may differ from the
certificate chain as served by the Known Pinned Host, of course.) It
is provided as an array of strings; each string pem1, ... pemN is the
PEM representation of each X.509 certificate as described in
[I-D.josefsson-pkix-textual].
The known-pins are the Pins that the UA has noted for the Known
Pinned Host. They are provided as an array of strings with the
syntax:
known-pin = token "=" quoted-string
Figure 6: Known Pin Syntax
As in Section 2.4, the token refers to the algorithm name, and the
quoted-string refers to the base 64 encoding of the SPKI Fingerprint.
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Pinning public keys helps hosts strongly assert their cryptographic
identity even in the face of issuer error, malfeasance or compromise.
But there is some risk that a host operator could lose or lose
control of their host's private key (such as by operator error or
host compromise). If the operator had pinned only the key of the
host's end entity certificate, the operator would not be able to
serve their web site or application in a way that UAs would trust for
the duration of their pin's max-age. (Recall that UAs MUST close the
connection to a host upon Pin Failure.)
Therefore, there is a necessary trade-off between two competing
goods: pin specificity and maximal reduction of the scope of issuers
on the one hand; and flexibility and resilience of the host's
cryptographic identity on the other hand. One way to resolve this
trade-off is to compromise by pinning to the key(s) of the issuer(s)
of the host's end entity certificate(s). Often, a valid certificate
chain will have at least two certificates above the end entity
certificate: the intermediate issuer, and the trust anchor.
Operators can pin any one or more of the public keys in this chain,
and indeed could pin to issuers not in the chain (as, for example, a
Backup Pin). Pinning to an intermediate issuer, or even to a trust
anchor or root, still significantly reduces the number of issuers who
can issue end entity certificates for the Known Pinned Host, while
still giving that host flexibility to change keys without a
disruption of service.
4.1. Maximum max-age
As mentioned in Section 2.3.2, UAs MAY cap the max-age value at some
upper limit. There is a security trade-off in that low maximum
values provide a narrow window of protection for users who visit the
Known Pinned Host only infrequently, while high maximum values might
potentially result in a UA's inability to successfully perform Pin
Validation for a Known Pinned Host if the UA's noted pins and the
Host's true pins diverge.
Such divergence could occur for several reasons, including: UA error;
Host operator error; network attack; or a Known Pinned Host that
intentionally migrates all pinned keys, combined with a UA that has
noted true pins with a high max-age value and has not had a chance to
observe the new true pins for the Host. (This last example
underscores the importance for Host operators to phase in new keys
gradually, and to set the max-age value in accordance with their
planned key migration schedule.)
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There is probably no ideal upper limit to the max-age directive that
would satisfy all use cases. However, a value on the order of 60
days (5184000 seconds) may be considered a balance between the two
competing security concerns.
4.2. Using includeSubDomains Safely
It may happen that Pinned Hosts whose hostnames share a parent domain
use different Valid Pinning Headers. If a Host whose hostname is a
parent domain for another Host sets the includeSubDomains directive,
the two Hosts' pins may conflict with each other. For example,
consider two Known Pinned Hosts, example.com and
subdomain.example.com. Assume example.com sets a Valid Pinning
Header such as this:
Public-Key-Pins: pin-sha1="ABC..."; pin-sha1="DEF..."; includeSubDomains
Figure 7: example.com Valid Pinning Header
Assume subdomain.example.com sets a Valid Pinning Header such as
this:
Public-Key-Pins: pin-sha1="GHI..."; pin-sha1="JKL..."
Figure 8: subdomain.example.com Valid Pinning Header
Assume a UA that has not previously noted any pins for either of
these Hosts. If the UA first contacts subdomain.example.com, it will
note the pins in the Valid Pinning Header, and perform Pin Validation
as normal on subsequent conections. If the UA then contacts
example.com, again it will note the pins and perform Pin Validation
on future connections. However, if the UA happened to first
example.com before subdomain.example.com, the UA would, due to
example.com's use of the includeSubDomains directive, attempt to
perform Pin Validation for subdomain.example.com using the SPKI
hashes ABC... and DEF..., which are not valid for the certificate
chains subdomain.example.com (which uses certificates with SPKIs
GHI... and JLK...). Thus, depending on the order in which the UA
observes the Valid Pinning Headers for hosts example.com and
subdomain.example.com, Pin Validation might or might not fail for
subdomain.example.com, even if the certificate chain the UA receives
for subdomain.example.com is perfectly valid.
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Thus, Pinned Host operators must use the includeSubDomains directive
with care. For example, they may choose to use overlapping pin sets
for hosts under a parent domain that uses includeSubDomains, or to
not use the includeSubDomains directive in their effective-second-
level domains, or to simply use the same pin set for all hosts under
a given parent domain.
4.3. Backup Pins
The primary way to cope with the risk of inadvertent Pin Failure is
to keep a Backup Pin. A Backup Pin is a fingerprint for the public
key of a secondary, not-yet-deployed key pair. The operator keeps
the backup key pair offline, and sets a pin for it in the Public-Key-
Pins header. Then, in case the operator loses control of their
primary private key, they can deploy the backup key pair. UAs, who
have had the backup key pair pinned (when it was set in previous
Valid Pinning Headers), can connect to the host without error.
Because having a backup key pair is so important to recovery, UAs
MUST require that hosts set a Backup Pin. (See Section 2.5.)
5. Privacy Considerations
Conforming implementations (as well as implementations conforming to
[RFC6797]) must store state about which domains have set policies,
hence which domains the UA has contacted. A forensic attacker might
find this information useful, even if the user has cleared other
parts of the UA's state.
More importantly, Hosts can use HSTS or HPKP as a "super-cookie", by
setting distinct policies for a number of subdomains. For example,
assume example.com wishes to track distinct UAs without explicitly
setting a cookie, or if a previously-set cookie is deleted from the
UA's cookie store. Here are two attack scenarios.
1. example.com can use report-uri and the ability to pin arbitrary
identifiers to distinguish UAs.
2.
1. example.com sets a Valid Pinning Header in its response to
requests. The header asserts the includeSubDomains
directive, and specifies a report-uri directive as well.
Pages served by the host also include references to
subresource https://bad.example.com/foo.png.
2. The Valid Pinning Header includes a "pin" that is not really
the hash of an SPKI, but is instead an arbitrary
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distinguishing string sent only in response to a particular
request. For each request, the Host creates a new, distinct
distinguishing string and sets it as if it were a pin.
3. The certificate chain served by bad.example.com does not pass
Pin Validation given the pin set the Host asserted in (1).
The HPKP-conforming UA attempts to report the Pin Validation
failure to the specified report-uri, including the
certificate chain it observed and the SPKI hashes it expected
to see. Among the SPKI hashes is the distinguishing string
in step (2).
3. example.com can use SNI and subdomains to distinguish UAs.
4.
1. example.com sets a Valid Pinning Header in its response to
requests. The header asserts the includeSubDomains
directive.
2. On a subsequent page view, the Host responds with a page
including the subresource https://0.fingerprint.example.com/foo.png, and the server responds using a certificate chain
that does not pass Pin Validation for the pin-set defined in
the Valid Pinning Header in step (1). The HPKP-conforming UA
will close the connection, never completing the request to
0.fingerprint.example.com. The Host may thus note that this
particular UA had noted the (good) pins for that subdomain.
3. example.com can distinguish 2^N UAs by serving Valid Pinning
Headers from an arbitrary number N distinct subdomains,
giving some UAs Valid Pinning Headers for some, but not all
subdomains (causing subsequent requests for
n.fingerprint.example.com to fail), and giving some UAs no
Valid Pinning Header for other subdomains (causing subsequent
requests for m.fingerprint.example.com to succeed).
6. IANA Considerations
This document has no actions for IANA. However, in the future there
may arise a need for a registry of extension directives (see
Section 2.1).
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When pinning works to detect impostor Pinned Hosts, users will
experience denial of service. UAs MUST explain the reason why, i.e.
that it was impossible to verify the confirmed cryptographic identity
of the host.
UAs MUST have a way for users to clear current pins for Pinned Hosts.
UAs SHOULD have a way for users to query the current state of Pinned
Hosts.
8. Acknowledgements
Thanks to Tobias Gondrom, Jeff Hodges, Ivan Krstic, Adam Langley,
Nicolas Lidzborski, SM, James Manger, Eric Rescorla, Paul Hoffman,
and Yoav Nir for suggestions and edits that clarified the text.
Thanks to Trevor Perrin for suggesting a mechanism to affirmatively
break pins ([pin-break-codes]).
9. What's Changed
[RFC EDITOR: PLEASE REMOVE THIS SECTION]
Added normative references for SHA, JSON, and base-64.
Added the Privacy Considerations section.
Changed non-normative pin generation code from Go to POSIX shell
script using openssl.
Changed max-max-age from SHOULD to MAY, and used the example of 60
days instead of 30.
Removed the section "Pin Validity Times", which was intended to be in
harmony with [I-D.perrin-tls-tack]. Now using max-age purely as
specified in [RFC6797].
Added new directives: includeSubDomains, report-uri and strict.
Added a new variant of the PKP Header: Public-Key-Pins-Report-Only.
Removed the section on pin break codes and verifiers, in favor the of
most-recently-received policy (Section 2.5).
Now using a new header field, Public-Key-Pins, separate from HSTS.
This allows hosts to use pinning separately from Strict Transport
Security.
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o Operators SHOULD get the backup public key signed by a different
(root and/or intermediary) CA than their primary certificate, and
store the backup key pair safely offline. The semantics of an
SPKI Fingerprint do not require the issuance of a certificate to
construct a valid Pin. However, in many deployment scenarios, in
order to make a Backup Pin operational the server operator will
need to have a certificate to deploy TLS on the host. Failure to
obtain a certificate through prior arrangement will leave clients
that recognize the site as a Known Pinned Host unable to
successfully perform Pin Validation until such a time as the
operator can obtain a new certificate from their desired
certificate issuer.
o It is most economical to have the backup certificate signed by a
completely different signature chain than the live certificate, to
maximize recoverability in the event of either root or
intermediary signer compromise.
o Operators SHOULD periodically exercise their Backup Pin plan
-\u002D an untested backup is no backup at all.
o Operators SHOULD start small. Operators SHOULD first deploy
public key pinning by using the report-only mode together with a
report-uri directive that points to a reliable report collection
endpoint. When moving out of report-only mode, operators should
start by setting a max-age of minutes or a few hours, and
gradually increase max-age as they gain confidence in their
operational capability.
Authors' Addresses
Chris Evans
Google, Inc.
1600 Amphitheatre Pkwy
Mountain View, CA 94043
US
Email: cevans@google.com
Chris Palmer
Google, Inc.
1600 Amphitheatre Pkwy
Mountain View, CA 94043
US
Email: palmer@google.com
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